In a switching power supply circuit, an exciting current is fed to a primary winding of a transformer thus causing energy stored in the transformer to be discharged as an output of a secondary output winding. The switching power supply circuit offers a stable power supply that is compact, lightweight and highly efficient, and is therefore utilized in power supply circuits such as those in battery chargers, AC adapters, and the like.
Conventionally, in this type of switching power supply circuit, an output voltage and current of a rectifying smoothing circuit of a secondary side are monitored such that excessively high power is not output from the rectifying smoothing circuit of the secondary side. The monitoring results are transmitted to a primary side using an insulated signal transmission element such as a photocoupler. On the primary side, an oscillating switching device is controlled so as to switch to ON and OFF in accordance with the transmission signal. Constant voltage control of the output voltage is executed by controlling an ON period (an energized period) and an OFF period of the exciting current fed to the primary winding (as in Japanese Patent Laid-Open Publication No. 2002-136116).
Hereinafter, a constant voltage control of the output voltage executed by a conventional switching power supply circuit 100, like the one described above, will be explained with reference to a circuit diagram shown in FIG. 8.
A direct current power supply 1 is an unstable power supply configured from a high voltage side terminal 1a and a low voltage side terminal 1b. A transformer 2 is configured from a primary winding 2a and a secondary output winding 2b. An oscillating switching device 3 is formed from a field effect transistor. Further, an Ip detection resistor 22 detects a primary winding current Ip that flows in the primary winding 2a. The oscillating switching device 3 is connected between an end of the primary winding 2a, and the low voltage side terminal 1b via the Ip detection resistor 22. The oscillating switching device 3 is switched ON and OFF with a predetermined cycle, by a switching control circuit 101 that is connected to a gate. Accordingly, the entire switching power supply circuit 100 oscillates.
A rectifying diode 4 and a smoothing capacitor 13, which are shown in a secondary side output of the transformer 2, configure a rectifying smoothing circuit. The diode 4 and the smoothing capacitor 13 rectify and smooth an output of the secondary output winding 2b, which is then output between the high voltage side output line 20a and a low voltage side output line 20b. An output monitoring circuit is provided between the output lines 20a and 20b. This output monitoring circuit monitors the output voltage and the output current and is configured from a voltage monitoring circuit and a current monitoring circuit. In the case that either the output voltage or the output current exceeds a respective predetermined reference voltage and reference current, the output monitoring circuit causes a photo coupler light-emitting device 35a, shown in the drawing, to emit light.
In the voltage monitoring circuit, voltage dividing resistors 30 and 31 are connected in series between the high voltage side output line 20a and the low voltage side output line 20b. A divided voltage of an output voltage is obtained from an intermediate tap point 32 and is inputted to an inverted input terminal of a differential amplifier 33a. Further, a voltage monitoring reference supply 34a is connected between a non-inverted input terminal of the differential amplifier 33a and the low voltage side output line 20b, and inputs a first comparative voltage to the non-inverted input terminal for comparison with the divided voltage of the output voltage. A reference voltage is set to a selected value by changing respective resistance values of the voltage dividing resistors 30 and 31, or the first comparative voltage of the voltage monitoring reference supply 34a. 
The photo coupler light-emitting device 35a is connected to an output side of the differential amplifier 33a. Further, the photo coupler light-emitting device 35a is connected to the high voltage side output line 20a via an electrical resistor 36, and is supplied with current from the drive power supply.
Moreover, a current detection resistor 43 is disposed in the low voltage side output line 20b in the current monitoring circuit, and one end of the current detection resistor 43 is connected to the inverted input terminal of the differential amplifier 33, and the other end is connected to the non-inverted input terminal via a current monitoring reference supply 34b. 
Accordingly, an output current that flows in the low voltage side output line 20b is indicated by a potential difference between both ends of the current detection resister 43. It can be determined whether this output current exceeds the predetermined reference current by comparison with a second comparative voltage of the current monitoring reference supply 34b in a differential amplifier 33b. A reference current is set to a selected value by changing a resistance value of the current detection resistor 43, or the second comparative voltage of the voltage monitoring reference supply 34b. An output side of the differential amplifier 33b is connected to a connection point of the output side of the differential amplifier 33a for monitoring the output voltage and the photo coupler light-emitting device 35a. 
Furthermore, the resistor 37a and the capacitor 38a, and the resistor 37b and the capacitor 38b, which are respectively connected in-series, act as alternating current negative feedback devices that cause operation of the differential amplifier 33a and the differential amplifier 33b, respectively, to be stable.
At the primary side of the transformer 2, a photo coupler light-receiving device 35b photo coupling with the photo coupler light-emitting device 35a is connected between the switching control circuit 101 and the low voltage side terminal 1b of the direct current power supply 1.
The switching control circuit 101 incorporates a variable reference supply 101a that outputs a variable voltage in accordance with a collector current of the photo coupler light-receiving device 35b that is configured from a phototransistor; a comparator 101b; an oscillator 101c; and an AND gate 101d. 
An inverted input of the comparator 101b is connected to a connection point of the oscillating switching device 3 and the Ip detection resistor 22, and a non-inverted input of the comparator 101b is connected to the variable reference supply 101a. Accordingly, a voltage by the Ip detection resistor 22a represented current Ip which flows in the primary winding 2a and a voltage output from the variable reference supply 101a represented light amount of a limit signal received by the photo coupler light-receiving device 35b from the photo coupler light-emitting device 35a are compared.
An output of the comparator 101b is input to the AND gate 101d along with an output of the oscillator 101c. Further, an output of the AND gate 101d is connected to a gate of the oscillating switching device 3.
With regard to the operation of the switching power supply circuit 100 configured in this way, when the variable reference supply 101a does not receive collector current from the photo coupler light-emitting device 35a, namely, in a normal operating state where the output is stable, a reference voltage Vset set to a predetermined value from the variable reference supply 101a is output to the non-inverted input of the comparator 101b. 
On the other hand, the voltage of the Ip detection resistor 22 that indicates the current Ip that flows in the primary winding 2a is input to the inverted input of the comparator 101b. The reference voltage Vset is compared to a primary winding current Ip that increases with the elapse of time once the oscillating switching device 3 has been switched to ON. Accordingly, the comparator 101b outputs “H” until the voltage indicating the primary winding current Ip reaches the reference voltage Vset, and then outputs “L” once the reference voltage Vset has been exceeded.
The oscillator 101c outputs a clock pulse that accords with an oscillation period T of the switching power supply circuit 100 to the AND gate 101d. As a result, the AND gate 101d outputs “H” when the clock pulse is “H” and the output of the comparator 101b is “H”, namely, when the voltage that indicates the primary winding current Ip has not reached the reference voltage Vset, and controls the oscillating switching device 3 to switch ON.
In contrast to this, when the output current increases past the reference current due to load connected between the high voltage side line 20a and the low voltage side line 20b, the voltage input to the inverted input terminal of the differential amplifier 33b rises. Thus, the potential difference between this voltage and the second comparative voltage is inverted and amplified, and reaches a potential that exceeds a light-emitting threshold value of the photo coupler light-emitting device 35a. 
Furthermore, even when the output voltage increases past the reference voltage due to load connected between the high voltage side line 20a and the low voltage side line 20b, the divided voltage input to the inverted input terminal of the differential amplifier 33b also rises. Thus, the potential difference between this voltage and the first comparative voltage is inversely amplified, and reaches a potential that exceeds the light-emitting threshold value of the photo coupler light-emitting device 35a. 
Accordingly, when either one of the output voltage and the output current exceeds the respective reference voltage or reference current, the photo coupler light-emitting device 35a emits a limit signal of an emitted light amount to the photo coupler light-receiving device 35b, in accordance with the respective exceeded amount.
When the photo coupler light-receiving device 35b receives the limit signal from the photo coupler light-emitting device 35a, the output voltage of the variable reference supply 101a reduces from the reference voltage Vset in accordance with the increase in the received light amount. Thus, the output of the comparator 101b is rapidly switched to “L”, as compared to the normal operation in which the reference voltage Vset is output.
As a result, the oscillating switching device 3 is switched on, a time T1 for which the primary winding 2a is excited is made shorter, and the energy stored in the transformer 2 reduces within one oscillation period. Accordingly, the output voltage or the output current, which respectively exceed the reference voltage or the reference current, spontaneously reduce, and become equal to or less than the reference voltage or the reference current.
Then, the photo coupler light-emitting device 35a stops emitting light and the photo coupler light-receiving device 35b no longer receives the limit signal. Accordingly, the oscillating switching device 3 once again repeats oscillation that is controlled according to the reference voltage Vset, and a stable output that accords with the power supplied to the load can be obtained.
In order for a voltage to be controlled to a constant by the constant voltage output control method, the switching power supply circuit 100 is provided with voltage dividing resistors 30 and 31 and a voltage monitoring reference supply 34a in a voltage monitoring circuit; a variable reference supply 101a that outputs a reference voltage Vset in a switching control circuit 101; and an Ip detection resistor 22 that is in-series with a primary winding 2a. However, as a result of variation of circuit constants of these circuit devices, variation of the integrated circuit itself when the switching control circuit 101 acts as an integrated circuit, and the like, a problem arises since stable and simple mass production of products having highly accurate constant voltage output characteristics is difficult.
Further, in case various output voltage characteristics of the switching power supply circuit are required, it becomes necessary to set each of the aforementioned circuit constants, and the like, or necessary to exchange circuit components, to adjust the characteristics. Accordingly, costs are increased due to factors such as an increase in time spent on intricate design and circuit component adjustment.
Moreover, an output voltage detection circuit is provided at a secondary side of a transformer 2. As a result, the number of components in the circuit is increased, thereby causing the overall circuit to become larger.
In addition, increase in the output voltage detected by the output voltage detection circuit of the secondary side of the transformer 2 is adjusted by control of the primary side. Accordingly, it is necessary to provide a photocoupler light-emitting device 35a, a photocoupler light-receiving device 35b, and so on, which leads to an increase in cost, as well as the circuit configuration becoming more complicated.